Image formation device

ABSTRACT

An image formation device changes the rotational rate of a polygon motor, i.e. the main-scanning rate, and selects one of pulse width conversion tables when an image formation is performed multiple times on a single sheet of transfer paper. The changing of the main-scanning rate is for preventing inconsistent sizes between a first recorded image and a second recorded image caused by shrinkage of transfer paper. The pulse width conversion tables allow the pulse widths to be modulated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/003,180filed Dec. 3, 2004, now U.S. Pat. No. 7,170,543, which claims priorityfrom Japanese Patent Application Nos. 2003-408071 filed Dec. 5, 2003 and2004-333196 filed Nov. 17, 2004, all of which are hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electro-photographic imageformation methods and devices such as digital copiers and laser beamprinters, and particularly, to an image formation method and devicewhich generates a pulse signal based on image data so as to form anelectrostatic latent image on a photosensitive element.

2. Description of the Related Art

In a typical image formation device using an electro-photographictechnique, a transferred toner image is fixed onto a sheet of transferpaper in order to form a permanent image on the paper. The toner imageis usually fixed onto the transfer paper using thermal fixing. Varioustypes of thermal-fixing techniques include hot air fixing, oven fixing,and more recently, heating roller fixing. In the heating-rollertechnique, a heating roller and a pressure roller are disposed inparallel to each other in the conveying path of the transfer paper, andthe transfer paper is conveyed through the two rollers while beingnipped therebetween. The heating roller fuses the toner, and at the sametime, the pressure roller presses the fused toner against the transferpaper so as to fix the toner on the transfer paper. As an alternative tosuch a structure, a pressure pad or a pressure belt is provided in placeof the pressure roller.

However, when using such a thermal-fixing technique to fix the tonerimage on the transfer paper, the heat causes the moisture in thetransfer paper to evaporate and thus causes the transfer paper toshrink. The degree of shrinkage may vary depending on, for example, thematerial of the transfer paper or the thickness of the paper.Furthermore, it is generally known from experience that it normallytakes about 15 to 20 minutes for the shrunken transfer paper to recoverits original size.

To perform double-sided recording on the transfer paper, athermal-fixing unit provided in the image formation device fixes a firsttoner image onto a first side of the transfer paper, and then thetransfer paper is turned over so that a second toner image can betransferred to a second side of the transfer paper. Subsequently, thethermal-fixing unit fixes the second toner image on the second side ofthe transfer paper. On the other hand, to combine two images on a singleside of the transfer paper, the thermal-fixing unit fixes a first tonerimage onto one side of the transfer paper, and then, without turningover the transfer paper, a second toner image is transferred to the sameside of the paper. Subsequently, the thermal-fixing unit fixes thesecond toner image onto the paper.

In both cases, the first formed image and the second formed image aredifferent in size because the transfer paper becomes smaller due toshrinkage after the first image is formed.

Japanese Patent Laid-Open No. 4-288560, for example, discloses atechnique for solving such a problem. Specifically, an optical sensor isdisposed upstream of a position where the transferring process isperformed in a conveying path of the transfer paper so as to detect thelongitudinal size and the lateral size of the transfer paper. Moreover,another optical sensor is disposed downstream of a thermal-fixing unitso as to similarly detect the longitudinal size and the lateral size ofthe transfer paper. Based on these detections, the shrinkage orexpansion proportion (i.e., change or ratio of the size before thermalfixing relative to the size after thermal fixing) of the transfer paperin both the longitudinal and lateral directions is determined. Based onthe determined shrinkage or expansion proportion, the scanning rate ofan optical scanner is controlled.

Japanese Laid-Open No. 10-149057 discloses a technique for reducing theworkload required in the above-mentioned technique. Specifically, anoptical sensor is disposed upstream of a position where the transferringprocess is performed in the conveying path of the transfer paper so asto detect the longitudinal size of a first sheet of transfer paperbefore or after the fixing process is performed. Based on the detection,the shrinkage or expansion proportion in the longitudinal direction ofthe first sheet of transfer paper is determined. Accordingly, thescanning rate of an optical scanner is controlled for the second sheetof transfer paper onward using the shrinkage or expansion proportion ofthe first sheet of transfer paper.

Image forming devices also use known modulation techniques to recordimages based on multiple-value image data for each pixel. One modulationtechnique is known as Pulse Width Modulation (PWM). In PWM, the width ofa pulse representing the light-emission time of a laser beam ismodulated for each pixel, while maintaining the intensity of the laserbeam. Another one is a technique in which the intensity of a laser beamis modulated while maintaining the light-emission time for each pixel.PWM is more commonly used since it provides a simpler control operationand more stable recording.

To prevent different sizes of formed images due to shrinkage of thetransfer paper, an image formation device that uses PWM not only changesthe scanning rate of the optical scanner based on the shrinkage orexpansion proportion of the transfer paper, but also changes themain-scanning rate (i.e. the rotational rate of a polygon motor) of alaser beam and the sub-scanning rate (i.e. the rotational rate of aphotosensitive element) of a laser beam with respect to thephotosensitive element.

However, when the main-scanning rate of the laser beam is changed, thedensity level of each electrostatic latent image segment formed on thephotosensitive element also changes. This is because the light-emissiontime is modulated for each pixel without changing the intensity of theemitted laser beam in the PWM technique, and therefore, when themain-scanning rate is changed, the amount of incident laser beam perunit area on the photosensitive element changes even if the recordingprocesses are performed based on the same image data having the samedensity levels.

For example, when forming electrostatic latent image segments based onimage data items having the same density level at a regular interval, ifthe main-scanning rate of the laser beam is lowered, the interval of theelectrostatic latent image segments becomes small, thus causing the areaof each latent image segment to become smaller. In such a case, if thepulse signal, i.e. pulse width, is not changed, the light-emission timeof the laser beam, namely, the amount of incident laser beam, remainsthe same, meaning that the same amount of laser beam is emitted to thesmaller area of each latent image segment, and therefore, the amount ofincident laser beam per unit area on the entire latent image isincreased. As a result, the density level of the entire electrostaticlatent image is increased.

SUMMARY OF THE INVENTION

The present invention is directed to an image formation device capableof resolving one or more disadvantages of conventional image formationdevices. Thus, one advantage of the present invention is that an imageformation device is disclosed that can prevent inconsistent image sizesfrom being formed due to shrinkage or expansion of transfer paper whenmultiple images are formed on a single transfer paper.

According to a first aspect of the present invention, an image formationdevice is provided. Such an image formation device performs an imageformation process by generating a pulse signal corresponding to imagedata; turning on/off a laser beam emitted towards a photosensitiveelement based on the pulse signal so as to form an electrostatic latentimage on the photosensitive element; developing the electrostatic latentimage; transferring the latent image to transfer paper; and fixing thetransferred image on the transfer paper. The image formation deviceincludes an obtainer for obtaining shrinkage or expansion information ofthe transfer paper before or after the image is fixed onto the transferpaper; a changer for changing a scanning rate of the laser beam based onthe shrinkage or expansion information obtained by the obtainer if theimage formation process is to be performed again on the same paperalready having the image fixed thereon; and a converter for changing apulse width for each pixel of the image data based on the scanning ratechanged by the changer when the pulse signal corresponding to the imagedata is generated.

According to a second aspect of the present invention, an imageformation device is provided, which performs an image formation processby generating a pulse signal corresponding to image data; turning on/offa laser beam emitted towards a photosensitive element based on the pulsesignal so as to form an electrostatic latent image on the photosensitiveelement; developing the electrostatic latent image; transferring thelatent image to transfer paper; and fixing the transferred image on thetransfer paper. The image formation device of the second aspect includesa changer for changing a scanning rate of the laser beam emitted towardsthe photosensitive element when the image formation process is to beperformed on both faces of the same transfer paper such that a firstlatent-image formation process for a front face of the transfer paperand a second latent-image formation process for a back face of thetransfer paper are to be performed at different scanning rates; and aconverter for changing an intensity of the laser beam based on thescanning rate changed by the changer such that the intensity of thelaser beam emitted towards the front face is different from theintensity of the laser beam emitted towards the back face.

According to these structures, even if the main-scanning rate is changedin order to prevent inconsistent sizes of formed images, such an imageformation device can compensate for the difference in the density levelof the subsequent image, which may be caused by the change in themain-scanning rate.

The above and other features, and advantages of the present inventionwill become more apparent from the following detailed description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an electro-photographic digitalcopier according to a first embodiment of the present invention;

FIG. 2 is a top view of an operating panel provided in theelectro-photographic digital copier according to the first embodiment;

FIG. 3 illustrates an image formation process from a photo-electricconversion step to a latent-image formation step according to the firstembodiment;

FIG. 4 illustrates the mechanical structure of a photo-control unitaccording to the first embodiment;

FIG. 5 is a block diagram illustrating the electrical structure of thephoto-control unit according to the first embodiment;

FIG. 6 illustrates the switching process of PWM conversion tablesaccording to the first embodiment;

FIG. 7 is a block diagram illustrating the electrical structure of aphoto-control unit according to a second embodiment of the presentinvention;

FIG. 8 is a block diagram illustrating the electrical structure of thephoto-control unit according to a third embodiment;

FIG. 9 is a detailed diagram illustrating the internal structure of alaser driver; and

FIGS. 10A and 10B each illustrate the relationship between a tonerdevelopment range and an energy distribution of an intensity of a laserbeam.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will now be described below with reference to theaccompanying drawings showing an embodiment thereof. In the drawings,elements and parts which are identical throughout the views aredesignated by identical reference numerals, and duplicate descriptionthereof is omitted.

FIG. 1 is a cross-sectional view of an electro-photographic digitalcopier according to a first embodiment of the present invention.Operation of the digital copier will also be described with reference toFIG. 3, which illustrates an image formation process from aphoto-electric conversion step to a latent-image formation stepperformed by the digital copier.

Among other components, the digital copier according to the presentembodiment includes a scanner 310 and a printer 330. The scanner 310includes a document feeder 1 which feeds original documents disposedthereon one by one to a glass base 2. Moreover, the scanner 310 furtherincludes a scanner unit 4 which contains a lamp 3 and a scanning mirror5. While the lamp 3 is turned on, the scanner unit 4 is capable ofmoving back and forth in the horizontal direction in FIG. 1 so as tophotoscan an image of each document disposed on the glass base 2. Inthis case, the light reflected from the document, i.e. image light,travels through scanning mirrors 5, 6, and 7 and is focused by a lens 8.Referring to FIG. 3, the image light is then input to an image sensor101. The image sensor 101 performs photo-electric conversion on theinput image light and outputs an analog image signal corresponding tothe image light. The analog image signal is then converted to a digitalsignal by an AD converter 102, and the digital signal is output to animage processor 320 as image data indicating the density levels.

The image processor 320 includes a shading compensator 103 whichcompensates for read errors by the scanner 310 with respect to the imagedata input from the AD converter 102. Moreover, the image processor 320further includes an image-processing circuit 104 which performs imageprocessing, such as γ conversion, on the image data. Subsequently,referring to FIG. 1, the image data is input to a photo-control unit 10in the printer 330 as multiple-value image data indicating multiplegray-scale levels.

Referring to FIGS. 1 and 3, the photo-control unit 10 converts the imagedata input from the image processor 320 to a pulse signal via a pulsewidth modulator (PWM) 105. The pulse signal is then sent to asemiconductor laser unit 106, specifically, a laser driver 114 shown inFIG. 5, where a laser beam to be emitted towards a photosensitiveelement 11 is turned on and off. When the laser beam enters thephotosensitive element 11, an electrostatic latent image i is formed onthe photosensitive element 11. Developer units 12 and 13 then developthe electrostatic latent image i into a toner image having predeterminedcolors.

In synchronization with the formation of the electrostatic latent imagei, a sheet of transfer paper is picked up from one of sheet trays 14 and15, and is conveyed to a transfer/separator unit 16 via a registrationroller 25. The transfer/separator unit 16 transfers the toner image onthe photosensitive element 11 to the transfer paper. The transfer paperhaving the toner image thereon is released from the photosensitiveelement 11 by the transfer/separator unit 16 and is delivered to athermal-fixing unit 17 where the toner image is fixed onto the transferpaper. Generally, the transfer paper is then conveyed through dischargerollers 18 so as to be discharged to a collection tray 20.

For double-sided recording, after the transfer paper advances past adischarge sensor 19, the discharge rollers 18 are rotated in a directionopposite to the discharge direction and a flapper 21 is shifted upward.Thus, the transfer paper having a first toner image fixed thereon isconveyed through conveying paths 22 and 23 and is delivered to anintermediate tray 24 in a state where the transfer paper is not turnedover. The transfer paper is then conveyed from the intermediate tray 24to the transfer/separator unit 16 during which process the transferpaper is turned over. Thus, a second toner image is transferred to thereverse surface of the transfer paper.

On the other hand, for multilayer recording, the flapper 21 is liftedupward before the transfer paper reaches the discharge sensor 19. Thus,the transfer paper having a first toner image fixed thereon is conveyedthrough the conveying paths 22 and 23 and is delivered to theintermediate tray 24 in a state where the transfer paper is turned over.The transfer paper in the intermediate tray 24 is then conveyed to thetransfer/separator unit 16, during which process, the transfer paper isturned over again. Thus, a second toner image is transferred to the samesurface of the transfer paper having the first toner image thereon suchthat the second toner image is disposed over the first toner image in amultilayer manner.

An optical sensor 26 is disposed upstream of the photosensitive element11 so as to measure the longitudinal size of the transfer paper, namely,the length of the paper in the sub-scanning direction. The opticalsensor 26 is further described with reference to FIG. 5.

FIG. 2 is a top view of an operating panel 5000 provided in the digitalcopier shown in FIG. 1.

Referring to FIG. 2, the operating panel 5000 includes a power switch5001 for turning the digital copier on and off; a reset key 5002 forresetting each mode to a standard mode during a stand-by state; a startkey 5003 for starting a copy operation; and a clear key 5004 fordeleting preset values.

The operating panel 5000 further includes an ID key 5005 which allowscopy operation for only a certain operator and restricts the use forother operators unless a proper identification code is input; a stop key5006 for pausing or aborting a copy operation; a help key 5007 used whenchecking the content of each function; up and down cursor keys 5008 and5009 for moving the cursor upward or downward in each function setupscreen; and left and right cursor keys 5011 and 5010 for moving thecursor left or right in each setup screen.

Furthermore, the operating panel 5000 further includes an OK key 5012for determining the content displayed on each function setup screen on adisplay 5052; an enter key 5013 for performing the content displayed atthe lower right corner of each function setup screen on the display5052; a reduce key 5014 for changing the current unit size to a smallerunit size; a direct key 5015 for direct copying; and an enlarge key 5016for changing the current unit size to a larger unit size.

Moreover, the operating panel 5000 further includes a tray selection key5017 for selecting one of the trays containing the transfer paper forthe copy operation; a first density selection key 5018 for reducing thecopy-density level; an AE key 5019 for automatic adjustment of thecopy-density level with respect to the density level of the originaldocument; a second density selection key 5020 for raising thecopy-density level; and a sort key 5021 for selecting an operation modefor a sorting function.

The operating panel 5000 further includes a preheat key 5022; a cut-inkey 5023 for performing interruption-copying; numerical keys 5024 fornumerical inputs; and a marker key 5025 for selecting a trimmingoperation, a masking operation, or a partial-treatment operation, suchas contouring, screening, shading, or a negative-positive operation. Theoperating panel 5000 further includes a paper-type input key 5026 forinputting the type of paper contained in each tray (i.e. paper material,thin paper, plain paper, thick paper); a color-delete key 5027 fordeleting a certain color; a picture-quality key 5028 for setting thepicture quality; a negative-positive key 5029 used when anegative-positive operation is performed; an image-create key 5030 whichis pressed when performing contouring, screening, shading, italicizing,image mirroring, or image repeating; and a trimming key 5031 forselecting a certain area in an image so as to trim the selected area.

The operating panel 5000 further includes a masking key 5032 forselecting a certain area in an image so as to mask the selected area;and a partial-treatment key 5033 for selecting a certain area of animage and performing a partial treatment operation, such as contouring,screening, shading, or a negative-positive operation, on the selectedarea. The operating panel 5000 also includes a binding key 5035 forcreating a binding margin on one side of the paper; a frame-erase key5034 for erasing the current frame based on a selected mode such as asheet-frame erasing mode in which a new frame is made based on thesheet-size, a document-frame erasing mode in which a new frame is madebased on the size of the original document, and a book-frame erasingmode in which a new frame is made and a blank space is added in thecentral portion based on the size of the double-spread opened pages of abook. Note that the size of the new frame is selectable.

The operating panel 5000 further includes a shift key 5036 for shiftingthe position of an image to be formed with respect to the transferpaper. In detail, the position of an image can be shifted in the upward,downward, left, and right directions, towards the center of the paper,towards one of the corners of the paper, or to a selected position onthe paper. Furthermore, a zoom key 5037 is provided for setting themagnification of the copying image by a unit of 1% in a range of 25% to400%. Moreover, such magnification can be set individually for bothmain-scanning and sub-scanning directions. The operating panel 5000further includes an auto-magnification key 5038 which automaticallyenlarges or reduces the size of the image to be formed based on the sizeof the transfer paper. Moreover, such an automatic magnification can beset individually for both the main-scanning and the sub-scanningdirections.

Furthermore, a multi-page enlargement key 5039 is provided for enlargingan image of a single original document and then copying the image tomultiple sheets of transfer paper. A reduce-layout key 5040 is providedfor reducing images of multiple sheets of original documents and thencopying the reduced images to a single sheet of transfer paper. Theoperating panel 5000 is further provided with a multi-page key 5043 fordividing the copy region on the glass base 2 into two halves andautomatically copying images of two sheets of original documents in acontinuous manner (continuous multi-page copying, continuousdouble-sided copying).

The operating panel 5000 further includes a double-side key 5044 forperforming double-sided recording on a single sheet of transfer paper(single-side to double-side (original to copy), multi-page todouble-side (original to copy), double-side to double-side). Amultilayer key 5045 is provided for recording images of multiple sheetsof original documents onto a single side of transfer paper in amultilayered manner (multilayer copy, multi-page multilayer copy). Theoperating panel 5000 further includes an MC key 5046 which is pressedwhen using a memory card; a projector key 5047 which is pressed whenusing a projector; and a printer key 5048 for setting modes when usingthe printer.

Also included on the operating panel 5000 is a mixed-document-size key5050 used when various sizes of documents are mixed in the documentfeeder 1; a mode-memory key 5051 for storing various types of copy modesor for reading and setting one of the stored copy modes. The display5052 displays the selected mode of the copier, the number of sheets tobe copied, the magnification of the copying image, and the selected sizeof the transfer paper. Moreover, when the copy mode is being set, thedisplay 5052 displays the content of the mode being set.

The operating panel 5000 further includes a system preheat key 5053 forturning a system preheat mode either on or off. Specifically, during thesystem preheat mode, a control unit for the copy operation is shut offwhile only the external interface and its peripheral circuit is inoperation. The operating panel 5000 includes a power display portion5054 which is lit when the power switch 5001 is turned on and which isturned off when the power switch 5001 is off.

FIG. 4 illustrates the mechanical structure of the photo-control unit10. In FIG. 4, a laser beam emitted from the semiconductor laser unit106 is substantially collimated by a collimator lens 107 and an aperture108. The collimated beam having a predetermined beam diameter thenenters a rotatable polygon mirror 109. The polygon mirror 109 is rotatedby a polygon motor 110 in a direction indicated by an arrow in FIG. 4 atan equiangular rate. Based on the rotation of the polygon mirror 109,the laser beam incident on the polygon mirror 109 is deflected such thatthe traveling direction of the laser beam changes at an equiangularrate. The deflected laser beam enters the photosensitive element 11 viaan f-θ lens unit 111 and is photoscanned across the photosensitiveelement 11. In this case, the f-θ lens unit 111 compensates for changesin the main-scanning rate caused by the differences in the opticaldistance of the laser beam deflected by the polygon mirror 109 to thephotosensitive element 11 so that the main-scanning rate can be setconstant.

A polygon-motor driver 112 controls the rotational rate of the polygonmotor 110 based on a rate-switching signal 431, that is, a targetscanning-rate signal, input from a CPU 117, which will be furtherdescribed below with reference to FIG. 5. When the rotational rate ofthe polygon motor 110 reaches the target scanning-rate, thepolygon-motor driver 112 outputs a rate-maintaining signal 432 to theCPU 117. Alternatively, instead of outputting the target scanning-ratesignal to the polygon-motor driver 112, the CPU 117 may directly outputa rate-adjustment value corresponding to the standard rotational rate.

A beam detector 113 is provided for detecting laser beams from thepolygon mirror 109. When a laser beam is detected by the beam detector113 and after a predetermined time period from the detection, thePWM-converted laser beam corresponding to the image data is photoscannedin the main-scanning direction.

FIG. 5 is a block diagram illustrating an electrical structure of thephoto-control unit 10.

In FIG. 5, the image data of one main scanning line processed by theimage processor 320, as described above, is stored in a line memory 115and is then supplied to the PWM 105 in a main controller 116. The PWM105 contains a plurality of PWM conversion tables T. Each of the PWMconversion tables T stores pulse widths corresponding to multiple valuesincluded in image data, the multiple values corresponding to multiplegray-scale levels. Moreover, the pulse width for a pixel correspondingto one image data item (i.e. density value) in one PWM conversion tableT is different from that of another PWM conversion table T. Furthermore,the PWM conversion tables T are provided in a non-volatile memoryportion of the main controller 116.

Based on the main-scanning rate, that is, the rotational rate of thepolygon motor 110, the CPU 117 selects one of the PWM conversion tablesT. The process for such a selection for the PWM conversion tables T willbe described later in detail. The CPU 117 sends a command signal to thePWM 105 so as to convert the image data to a PWM signal based on theselected PWM conversion table T. Then, based on the PWM signal outputfrom the PWM 105, the laser driver 114 turns on and off the laser beamemitted from the semiconductor laser unit 106.

Furthermore, based on a signal from the optical sensor 26, the CPU 117determines the shrinkage or expansion proportion of transfer paper in acase where double-sided recording or multilayer recording is to beperformed. Based on the determined shrinkage or expansion proportion,the CPU 117 controls the polygon-motor driver 112 to change therotational rate of the polygon motor 110. Then, based on the changedrotational rate, the CPU 117 selects an appropriate table from the PWMconversion tables T. To run this operation, the CPU 117 implements acontrol program included in a ROM 118 while utilizing a RAM 119 as awork space.

The operation for changing the rotational rate of the polygon motor 110and the operation for selecting one of the PWM conversion tables T willnow be described.

After advancing through the thermal-fixing unit 17, a transfer paper hasshrunk due to water evaporation. The shrinkage proportion of thetransfer paper is detected by the optical sensor 26. In detail, theoptical sensor 26 disposed upstream of the photosensitive element 11 isprovided with a light emitter and a photo-acceptor. The CPU 117 measuresthe time period between a point in which light is emitted from the lightemitter and a point in which the light reflected from the transfer paperis received by the photo-acceptor. The CPU 117 then calculates theproduct of the measured time period value multiplied by the conveyingrate value of the transfer paper so as to determine the longitudinalsize of the transfer paper, namely, the length of the paper in thesub-scanning direction.

Subsequently, in double-sided recording or multilayer recording, the CPU117 measures the longitudinal size L1 of the transfer paper before thefirst recording process, and moreover, the CPU 117 measures thelongitudinal size L2 of the transfer paper between the end of the firstrecording process and the start of the second recording process so as tocalculate the ratio of L2 to L1. Thus, a shrinkage proportion (ratio orchange) ΔL in the longitudinal direction of the transfer paper isdetermined. Although the transfer paper may shrink both in thelongitudinal and lateral directions, the shrinkage proportion isconsidered to be substantially equivalent in the two directions.Therefore, in the first embodiment, to simplify the structure and thecontrol operation, the shrinkage or expansion proportion in the lateraldirection is the same as the shrinkage or expansion proportion ΔL in thelongitudinal direction.

Accordingly, since the size of the transfer paper is different betweenthe first recording process and the second recording process due toshrinkage or expansion, if the rotational rate of the polygon motor 110,i.e. the main-scanning rate, and the rotational rate of thephotosensitive element 11, i.e. the sub-scanning rate, are not to bechanged, the size of the first recorded image and the size of the secondrecorded image will be different. Consequently, based on the shrinkageor expansion proportion ΔL in the lateral direction of the transferpaper, the CPU 117 calculates a main-scanning rate value thatcompensates for the change in the lateral size of the recorded image.Thus, the rotational rate of the polygon motor 110 is switched to thecalculated main-scanning rate according to which main-scanning isperformed during the second recording process. Moreover, based on theshrinkage or expansion proportion ΔL in the longitudinal direction ofthe transfer paper, the CPU 117 also calculates a sub-scanning ratevalue that compensates for the change in the longitudinal size of therecorded image. Thus, the rotational rate of a motor (not shown) forrotating the photosensitive element 11 is switched to the calculatedsub-scanning rate according to which sub-scanning is performed duringthe second recording process.

The light-emission time is modulated by using PWM for each pixel withoutchanging the intensity of the emitted laser beam. For this reason, whenthe main-scanning rate is switched, as described above, the amount ofincident laser beam per unit area on the photosensitive element 11changes even if the recording processes are performed based on the sameimage data having the same density levels. Consequently, this results indifferent density levels of the images formed.

For example, if an image is to be formed on a sheet of shrunken transferpaper and the main-scanning rate of the laser beam is lowered so as toprevent the image from being over-sized, the density level of the imageincreases. As shown in the upper section of FIG. 6, to prevent such adensity level increase, the CPU 117 selects a PWM conversion table Tthat is based on a pulse width conversion rule γ1. In detail, withrespect to image data having a predetermined density level, such a pulsewidth conversion rule γ1 allows the pulse width to be narrower for everypixel. In contrast, if an image is to be formed on a sheet of expandedtransfer paper and the main-scanning rate of the laser beam is increasedso as to prevent the image from being under-sized, the density level ofthe image decreases. As shown in the lower section of FIG. 6, to preventsuch a density level decrease, the CPU 117 selects a PWM conversiontable T that is based on a pulse width conversion rule γ2. In detail,with respect to image data having a predetermined density level, such apulse width conversion rule γ2 allows the pulse width to be wider forevery pixel.

To switch the rotational rate of the polygon motor 110, i.e. themain-scanning rate, in a multilevel manner, the number of PWM conversiontables T, i.e. the number of the rules, may be increased. Furthermore,for continuously performing double-sided recording or multilayerrecording on multiple sheets of transfer paper of the same paper type,the shrinkage or expansion proportion calculated for the first sheet oftransfer paper may be used for the second and additional transferpapers. This can reduce the workload on the CPU 117. This conceptapplies to a second embodiment of the present invention, which will bedescribed later.

According to the first embodiment, when image formation is to beperformed multiple times on a sheet of transfer paper having an imagefixed thereon, the rotational rate of the polygon motor 110, i.e. themain-scanning rate, is switched so as to prevent inconsistent sizes ofthe formed images due to shrinkage or expansion of the transfer paper.Moreover, an appropriate PWM conversion table T is selected forperforming a pulse width conversion that compensates for the differencein density levels between the images caused by the switching of themain-scanning rate. Accordingly, this prevents inconsistent densitylevels of images and thus allows high-quality images to be formed.

Referring to FIG. 7, a second embodiment of the present invention willnow be described. The second embodiment takes a different approach indetecting the shrinkage/expansion proportion of the transfer paper, asshown in FIG. 7.

According to the second embodiment, the operating panel 5000 is providedwith the paper-type input key 5026 for inputting the type of transferpaper used, i.e. paper material, thin paper, plain paper, thick paper.Furthermore, the digital copier contains a hygrothermal sensor 120 fordetecting the temperature and the humidity. Moreover, the nonvolatilememory portion of the main controller 116 includes a shrinkage orexpansion proportion table T1 in which shrinkage or expansion proportionvalues are stored. Specifically, such shrinkage or expansion proportionvalues are values of transfer paper per unit length just before or afterpassing through the thermal-fixing unit 17, and are classified accordingto different paper types, temperatures, and humidity levels whileassuming that the shrinkage or expansion proportion is the same in boththe longitudinal and lateral directions.

According to such a structure, based on the paper type input via thepaper-type input key 5026 and the temperature and humidity detected bythe hygrothermal sensor 120, the CPU 117 selects a correspondingshrinkage or expansion proportion value of transfer paper per unitlength from the shrinkage or expansion proportion table T1. Thus,similar to the first embodiment, the CPU 117 changes the rotational rateof the polygon motor 110, i.e. the main-scanning rate, based on theselected shrinkage or expansion proportion value so as to preventinconsistent sizes between the first recorded image and the secondrecorded image onward which may be caused by shrinkage or expansion ofthe transfer paper. Moreover, the CPU 117 selects an appropriate PWMconversion table T for performing a pulse width conversion thatcompensates for the difference in density levels between the imagescaused by the switching of the main-scanning rate.

If the shrinkage or expansion proportion is different for thelongitudinal and lateral directions, the shrinkage or expansionproportion values in the shrinkage or expansion proportion table T1 maybe stored according to different paper types, temperatures, humiditylevels, and directions (i.e. the longitudinal and lateral directions).

Consequently, in the second embodiment, an appropriate shrinkage orexpansion proportion value is simply selected from a table and is notdetermined based on calculation. This reduces the workload on the CPU117 while still achieving the same effect as the first embodiment.

As described above, in the first and second embodiments, theinconsistent density levels of images are corrected by selecting anappropriate PWM conversion table corresponding to a change in therotational rate of the polygon motor 110 (main-scanning rate), which isbased on the shrinkage or expansion proportion of the transfer paper. Onthe other hand, referring to FIGS. 8 to 10B, according to a thirdembodiment of the present invention, an intensity of a laser beam isdirectly changed in order to correct the inconsistent density levels.

In other words, referring to FIG. 8, according to the third embodiment,the CPU 117 in the main controller 116 sends an intensity compensationsignal S409 to the laser driver 114 based on a shrinkage or expansionproportion of the transfer paper determined by the optical sensor 26. Inthis case, a PWM conversion table T in the PWM 105 is constantregardless of the main-scanning rate based on the shrinkage or expansionproportion of the transfer paper.

FIG. 9 illustrates the internal structure of the laser driver 114 andthe laser unit 106. The laser unit 106 defining an optical scanner oflaser beams includes a laser diode 404, and a photo-diode 405 disposedadjacent to the laser diode 404. Some components of a laser beam fromthe laser diode 404 are received by the photo-diode 405, and areamplified by a monitor amplifier 406. Thus, a signal S407 proportionalto the intensity of the emitted laser beam is obtained. Subsequently,the signal S407 is compared with a laser-beam target-intensity standardsignal Vt level preliminarily input in an APC (Automatic Power Control)controller 401. The APC controller 401 then sends an analog signal S408to a constant-current circuit 403 so that the emission power of thelaser diode 404 is set constant. Accordingly, after the intensity of thelaser beam is set, a switching circuit 402 is turned on and off based ona PWM signal output from the PWM 105 so as to control the emission ofthe laser diode 404.

Similar to the first embodiment, based on a signal from the opticalsensor 26, the CPU 117 calculates the shrinkage or expansion proportionΔL in the main-scanning direction of transfer paper for double-sidedrecording or multilayer recording. Then, a main-scanning rate thatcompensates for a change in the lateral size of the correspondingrecording image is determined. The rotational rate of the polygon motor110 is switched so as to perform the second recording process using thedetermined main-scanning rate. When the rotational rate (main-scanningrate) is switched, the intensity of the laser beam per unit area on thephotosensitive element becomes inconsistent even if the recordingprocess is performed based on image data having the same density level.This may result in inconsistent density levels of the image. Forexample, if the main-scanning rate of the laser beam is set lower so asto prevent an image from becoming larger when the image is recorded onshrunken transfer paper, the density level of the image becomes higher.In such a case, the preliminarily-set laser-beam target-intensitystandard signal Vt level corresponding to the image data having apredetermined density level is output from the CPU 117 via the intensitycompensation signal S409. The laser-beam target-intensity standardsignal Vt level is changed to a Vt′ level so that the output intensityof the laser diode 404 is lowered. Thus, referring to FIG. 10A, thelaser energy distribution (electrostatic latent image distribution) pereach dot on the exposed photosensitive element 11 in the main-scanningdirection becomes smaller. Consequently, the developed toner imagebecomes smaller with respect to a specific threshold of development.Thus, the density level of the developed image is compensated for withrespect to the shrunken transfer paper so as to achieve a consistentdensity level in a double-sided recording or a multilayer recording. Onthe other hand, if the main-scanning rate of the laser beam is sethigher so as to prevent an image from becoming smaller when the image isrecorded on expanded transfer paper, the laser-beam target-intensitystandard signal Vt level is output from the CPU 117 via the intensitycompensation signal S409 such that the Vt level is changed to a Vt″level. Thus, the output intensity of the laser diode 404 is increased.Referring to FIG. 10B, the laser energy distribution (electrostaticlatent image distribution) per each dot on the exposed photosensitiveelement 11 in the main-scanning direction becomes larger such that thedeveloped toner image becomes larger with respect to a specificthreshold of development. Thus, the density level of the developed imageis compensated for with respect to the expanded transfer paper so as toachieve a consistent density level in double-sided recording ormultilayer recording.

Accordingly, the third embodiment achieves substantially the same effectas the first and second embodiments by directly adjusting the intensityof the laser beam without having to change the PWM conversion table T.

Furthermore, although the optical sensor 26 of the first embodiment isused as means for calculating the shrinkage or expansion proportion oftransfer paper in the third embodiment described above, the paper-typeinput key 5026 and the hygrothermal sensor 120 of the second embodimentmay alternatively be used for calculating the shrinkage or expansionproportion.

Furthermore, the present invention is not limited to the aboveembodiments. For example, the present invention can be applied to arecording mode in which an image formation process including theimage-fixing step on a single sheet of transfer paper is performedmultiple times. Furthermore, in the first embodiment, in addition to theoptical sensor 26 that detects the longitudinal size of the transferpaper, an additional optical sensor for detecting the lateral size ofthe transfer paper may be provided. Such an additional optical sensormay be used to determine the shrinkage or expansion proportion in thelateral direction of the transfer paper. Moreover, the positioning ofsuch optical sensors is not limited to the upstream side of thephotosensitive element 11, and may be in any location consistent withthe principles and scope of the present invention.

Furthermore, by monitoring the change in the level of current flowingthrough a motor for driving the registration roller 25, for example, thetime period in which the registration roller 25 is in contact with thetransfer paper can be detected. Thus, based on this time period, thelongitudinal size of the transfer paper may be determined.

Furthermore, the present invention may include a modification where astorage medium (or a recording medium) storing software program codethat achieves the functions of the above embodiments is supplied to acomputer (CPU (central processing unit) and MPU (microprocessor unit))of a system or a device, such that the computer operates by reading theprogram code stored in the storage medium.

In this case, the program code read from the storage medium implementsthe functions of the above embodiments, and therefore, the storagemedium storing the program code may be included within the technicalscope of the present invention. Accordingly, the program code may beoperated by the computer to implement the functions of the aboveembodiments. Alternatively, in response to a command given by theprogram code, an operating system (OS) of the computer, for example, mayentirely or partially perform the actual process so as to implement thefunctions of the above embodiments.

The present invention may further include a modification where theprogram code read from the storage medium is stored in a memory includedin an enhancement card inserted into a computer or an enhancement unitconnected to a computer. In response to a command from the program code,a CPU, for example, provided in the enhancement card or the enhancementunit performs the overall or partial process, whereby the functions ofthe above embodiments may be implemented. Accordingly, when the presentinvention is applied to such a storage medium, the storage medium storesthe program code corresponding to the above-mentioned process.

While the present invention has been described with reference to whatare presently considered to be the embodiments, it is to be understoodthat the invention is not limited to the disclosed embodiments. On thecontrary, the invention is intended to cover various modifications andequivalent arrangements included within the spirit and scope of theappended claims. The scope of the following claims is to be accorded thebroadest interpretation so as to encompass all such modifications andequivalent structures and functions.

1. An image formation device which performs an image formation processon transfer paper by rotating a rotatable polygonal mirror having aplurality of reflection surfaces which reflect a laser beam from a lasersource to scan an image carrier, the image formation device comprising:a heat treating unit configured to heat-treat the transfer paper onwhich the image formation process is performed; a driving unit rotatablydriving the rotatable polygonal minor; a shrinkage/expansion informationobtaining unit configured to obtain shrinkage or expansion proportion ofthe transfer paper caused by the heat treatment; and a controlling unitconfigured to change the rotating speed of the driving unit and a pulsewidth for one pixel of image data, according to the shrinkage/expansionproportion obtained by the shrinkage/expansion information obtainingunit, when the image formation process is performed again on thetransfer paper which was already heat-treated.
 2. The image formationdevice according to claim 1, wherein the controlling unit changes thepulse width for one pixel for the image data by converting a table whichshows the relation of the image data formed on the image carrier and thepulse width of the driving sigual of the laser source.
 3. The imageformation device according to claim 1, wherein the shrinkage/expansioninformation obtaining unit includes a detecting unit configured todetect the size of the transfer paper and obtains theshrinkage/expansion proportion of the transfer paper based on the sizeof the transfer paper before and after a fixing process.
 4. The imageformation device according to claim 1, wherein the shrinkage/expansioninformation obtaining unit includes an input unit configured to input apaper quality of the transfer paper and obtains a shrinkage/expansionproportion of the transfer paper based on the paper quality input by theinput unit.